Hearing has evolved independently many times in the animal kingdom and is prominent in several insects and vertebrates for communication and predator detection. Among the insects, katyds The ears are unique in that they have evolved components of the outer, middle, and inner ear, analogous in their biophysical principles to the mammalian ear. In new research, paleontologists from the University of Lincoln and elsewhere reconstructed the geometries of the components of the outer ear and wings of Eomortoniellus handlirschi, an exceptionally well-preserved grasshopper fossilized in a 44-million-year-old piece of Baltic amber. they found that Eomortoniellus handlirschi It communicated at a maximum frequency of 32 kHz and demonstrated that the ear was biophysically tuned to this signal and to provide hearing with higher frequency ultrasound (over 80 kHz), probably for better predator detection. The results indicate that the evolution of the grasshopper’s single ear, with its broadband ultrasonic sensitivity and biophysical properties analogous to mammalian ears, emerged in the Eocene epoch.
reconstruction of Eomortoniellus handlirschi. Image credit: Charlie Woodrow.
About 44 million years ago, an adult male Eomortoniellus handlirschi It became trapped in pine resin, which hardened and encased the insect.
The amber specimen was discovered in 1936 in an area then known as East Prussia, Germany.
CT scans of the insect revealed that the sap had entered the grasshopper’s ear canal, located on the inside of its legs.
With the sap inside, the delicate structure of the insect’s ear was well preserved.
“This discovery would not have been possible without such a well-preserved grasshopper, highlighting the importance of museum collections in uncovering specimens like these,” said Dr. Charlie Woodrow, former Ph.D. Student at the University of Lincoln who is now a researcher at Uppsala University.
“This grasshopper was frozen in time at a crucial moment in the arms race between predators and echolocating insects.”
“Shortly before this animal was fossilized, bats had developed the ability to echolocate, which may have led grasshoppers to call at higher frequencies.”
“At the same time, their ears were adapting to hear the bats trying to hunt them.”
Evidence suggests that katydids developed the ability to send mating calls at increasingly higher frequencies to go unnoticed by their mammalian predators, until they switched to ultrasonic sound.
These sounds would not have been detected until the first bats evolved to use laryngeal echolocation about 52 million years ago.
“While grasshoppers were probably already exploring high frequencies, both to prevent eavesdropping and to develop greater signal diversity, bats gave them a new boost,” said Professor Fernando Montealegre-Zapata of the University of Lincoln.
“It may seem strange that grasshoppers continued to sing at these high pitches once they could be heard, but ultrasound dissipates quickly in the environment.”
“This ensures that a distant bat will not hear the grasshopper’s song, as the sound will be interrupted before it can be heard.”
“The morphological and physiological characters that these insects developed in response to mammalian predators, mainly bats, are the main current traits that define the entire family of bush crickets (Tettigoniidae), which also originated in the Eocene.”
Using models of how sound travels and how living grasshoppers produce sound, the authors were able to reconstruct the mating signal of Eomortoniellus handlirschi.
They calculated that the grasshopper probably heard sounds around 30 kHz best.
This suggests that the insect’s hearing evolved to better hear the calls of its species, giving it the best opportunity to match pairs for mating.
The researchers also found two other peaks in the grasshopper’s hearing, around 60 and 90 kHz.
This likely helped the insect tune into any nearby echolocation calls of early bats, which were around 40-65 Hz.
The insects’ ability to hear high frequencies would have been enhanced by grasshoppers’ pinnae, or earlobes in mammals.
Although it was only partially developed in Eomortoniellus handlirschiEvolution in the years following his entrapment in amber has allowed his family members to hear calls over 100 kHz.
“It is now important to identify more fossils to track these changes,” Dr Woodrow said.
“I think more adults of this species, or its close relatives, will appear, as many are sold online to private amber collections and preserved in public collections. “It just takes the right people to realize it and study it.”
The teams paper was published in the magazine Current biology.
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Charlie Woodrow et al. An Eocene insect could hear ultrasound and echolocation of bats of the same species. Current biology, published online November 13, 2023; doi:10.1016/j.cub.2023.10.040
